1. Field of the Invention
The present invention generally relates to a method for forming a metal gate. In particular, the present invention is directed to a method for forming a metal gate which is constructed from a recess substantially free of undercut. The recess which is substantially free of undercut is made by selective etching to remove an undoped dummy material without substantially damaging a doped dummy material, which avoids the problem of lateral erosion of the recess by the etching.
2. Description of the Prior Art
In the manufacturing procedures of semiconductor elements, some etching processes are usually used to define the locations of needed elements. For example, during the manufacturing procedures of static random access memory (SRAM), a pair of adjacent gate structures is needed to be formed. Generally speaking, in order to metal gates two consecutive etching processes are usually needed to respectively define the locations of the first metal gate structure and the second metal gate structure.
In the first etching process, generally a wet etching process, a mask is usually used to protect some regions from unwanted etching in order to restrict the extent of the etching by an etchant. As a matter of fact, the etchant will always inevitably carry out a collateral lateral etching in addition to the pre-determined vertical etching, in particular to remove the target material at the edge of the mask, in spite of the protection of the mask. As a result, such collateral lateral erosion will always lead to a void undercut under the edge of the mask. Such void undercut on one hand twists the shape of the needed recess, and on the other hand it also jeopardizes the performance of the adjacent semiconductor element.
Accordingly, a solution is still needed to remove the target material without substantially harming the adjacent material so as to form a perfect recess with no undercut present.